Patent GB-724836-A (GB)
Abstract: 724,836. Sparking plugs. GENERAL MOTORS CORPORATION. April 15, 1953 [May 1, 1952], No. 10266/53. Class 7 (4). In a low-tension sparking plug comprising electrodes having portions, such as a head 24, Fig. 1, and a flange 42, that are spaced to define a spark gap 44, and an electrical insulator 8 separating the electrodes and having a surface 38 which bridges the spark gap to form a discharge path, a layer of electrical conductive material, for example gaskets 50, 52 of annealed nickel or nickel alloy, is interposed between each of the electrode portions 24, 42, respectively, and the surface 38, the gaskets being wholly outside the spark gap. The insulator 8, Fig. 1, houses a centre wire 10 with a screw threaded or serrated inner portion 22 and a centre electrode 18, the latter being provided with tne head z4 and a bifurcated inner end, the prongs 26 of which are bent outwardly and over the rop of a ring 28 that retains the electrode 18 in the lower end of the insulator bore 12. A conductive seal 16 comprising a mixture of glass and a copper or nickel powder is bonded to the walls of bore 12, the centre wire 10 and the electrode 18. The flange 42 in the shell 6 serves both as an earth electrode and, in conjunction with an upper flange 46 and a washer 48, as means for securing the insulator in position ; the insulator surface 38 is provided with a semi-conductive coating. The gaskets 50, 52 have an annular gap therebetween which, is in alignment and of the same dimension as that between head 24 and electrode 42 ; the gaskets serve as gas seals and prevent electrode erosion. In a modification, the shell encloses the insulator and includes an upper insulator to centralize and support the centre wire whilst the earth electrode comprises an insert in the lower end of the shell, which insert encloses the lower and enlarged position of the insulator. In a further modification, the shell is L-shaped the two parts 112 and 114, Fig. 3 thereof being welded or brazed together ; the centre wire 118 is similarly constructed, the horizontal part 122 having a threaded portion 124 which is cemented or otherwise secured in an insulator 126 that is secured by a nut 128 in shell part 114. Metal washers 130 and 132 seal the assembly. The lower shell part 112 houses an insulator 136 the lower face of the latter being semi-conductive whilst the centre electrode 140 includes the head 142 and a threaded portion engaged by a member 144 by means of which the electrode is retained within the insulator bore 138. The centre wire 118 is brazed or welded into the electrode 140. A metal sleeve 150 is threaded in the shell part 112 and presses the insulatorcentre electrode assembly toward the earth electrode comprised by a shell flange 154. Metal washers 130 and 132 and 156 and 158 seal the centre wire and centre electrode assemblies, respectively. Gaskets 164 and 166 are provided as in Fig. 1 and the plug is mounted by flange 168. A groove 113 is provided in the shell so that upon passing an electric current therethrough the shell becomes plastic after which pressure is applied to the two ends of the shell to cause collapse of the grooved sections and exert pressure on washer 158 and gasket 166. A similar process is carried out with the electrode 140 before insertion of the insulatorcentre electrode assembly in the shell part 112 to apply pressure on washer 156 and gasket 164. The shell of Fig. 1 and the insert of the preceding embodiment are grooved for the same purpose. In another modification, the insulator 188, Fig. 4, is held in the insert 180, which acts as the earth electrode, by flanges 182 and 194 the latter being formed on a head member 174 threaded on the centre electrode 172. The insulator is coated 190 with a mixture of ceramic and conductive or semi-conductive material, such as metal powder metal oxides or carbon ; a second coating 206, 208 of nickel is also sprayed or otherwise provided on the vertical sides of the insulator, whilst gaskets 210, 212 are used as hitherto. The gaskets in all the foregoing embodiments may be replaced by nickel coatings e.g. 206, 208 the spark gap 214 being obtained by machining away coating at that point.